Massive MIMO is a candidate technology for 5G cellular networks. In a massive MIMO system, base stations (BSs) are equipped with more antennas than in conventional systems, e.g., each base station may have 20 to 100 antennas or more, thus a large number of users are served simultaneously using the multiuser MIMO techniques. Massive MIMO has advantages of being less affected by thermal noise and fast fading, simplified multiuser processing, and reduced transmit power and high sum-rates.
Channel state information (CSI) is critical in massive MIMO systems, and is used to separate data to different users through transmit/receive precoding. Conventionally, in a massive MIMO system that operates in a time division duplexing (TDD) mode, it is assumed that channel reciprocity holds, and consequently, uplink (UL) and downlink (DL) channels are assumed to be the same, which allows a base station to estimate DL channels to its associated users based on UL pilots, thus avoiding explicit CSI feedback. Channel reciprocity actually does not hold in massive MIMO systems. For example, in TDD-based massive MIMO systems, there are non-ideal hardware and calibration errors, and in frequency duplex division (FDD) based massive MIMO systems, different carrier frequencies are used on UL and DL. In a conventional approach for DL channel estimation in a massive MIMO system, a user estimates a DL channel based on DL training sequences received from its associated base station and sends DL CSI back to the base station for precoding design.